in COELENTERATA 39 



digestive cavity, close to the reproductive organs, there are 

 many little ciliated filaments, the gastric filaments, richly 

 beset with stinging cells, the function of which is to paralyse 

 any prey that may be swallowed alive. These do not occur 

 in any of the Hydro-medusae described in Chapter II. 



The nervous system also differs from that of the medusae of 

 the Hydrozoa, for, instead of a ring of nerve cells, there are 

 eight groups of such cells near the eight pairs of marginal lappets 

 where special sense organs are situated ; these latter consist 

 merely of little pigment masses or eye-spots, and also of small 

 pits, lined with sensory cells, known as olfactory pits, by means 

 of which the animal may detect the presence in the water of 

 things good to eat or the reverse ; little, however, is known 

 of the special functions of these structures. They are often 

 called "tentaculocysts," for they are thought to represent 

 modified tentacles ; they only occur in these large jelly-fish, 

 and form one of the distinguishing marks between them and 

 the medusae of the Hydrozoa, from which they also differ in 

 having no velum to the bell. 



In the Hydrozoa, the medusa is merely a specially 



ment P m dified individual, formed by budding on a colony 



of ordinary hydroid polyps, but in the Scyphozoa 



the medusa is the dominant phase ; the hydroid when present 



being a little insignificant structure, about \ an inch long, 



known as the Scyphistoma. 



This little hydroid develops in the late summer directly 

 from the fertilised egg. Even before leaving the parent jelly- 

 fish, the egg has developed into 

 a little oval hollow sac open at 

 one end, with a two-layered wall 

 covered with cilia, by means of 

 which it makes its way out of 

 the mouth of the jelly-fish into 

 the water. It swims freely for Fia i 8 ._ Aurelia aurita. Stages in 

 a short time (being known in the development of the hydroid. 

 this stage as the " planula "), ( After Korsch elt and Heider.) 

 but finally it becomes attached " Mouth and mo th - tube developing, 

 to some object in the water, loses its cilia and develops a 

 mouth as shown in Fig. 18, B and G. Tentacles then begin 

 to grow out round the mouth until there are sixteen arranged 

 at regular intervals, the mouth-cone is pushed outwards and 



